This invention relates to lighting systems, and more particularly, to a lighting system having a lamp dimmer which is controlled in response to changes in illumination over an area.
Conventional interior and outdoor lighting design has been approached from the standpoint of providing uniform illumination over an area. Typically, lighting systems have been designed to provide a recommended minimum level of illumination for a given area depending upon the activity contemplated for that area. The calculations made in designing conventional lighting systems take into consideration light-loss factors through the use of a so-called "maintenance factor." The maintenance factor accounts only for lamp lumen depreciation and luminaire dirt depreciation. The effect of the approach to lighting design of establishing a level of illumination over a specified area of some stated minimum maintained footcandles is that the systems initially produce considerably more footcandles than are really required.
In recent years, a new approach in lighting system design has emerged. Lighting designers are now designing in terms of uniform illumination relative to time as well as relative to space. This shift in design approach is largely due to the advances made in controlling the lumen output of high intensity discharge (HID) lamps. The approach of establishing uniform illumination relative to time has been implemented through the use of dimmer circuits. Dimming of HID lamps makes practical such concepts as preprogramed light levels to match area activities, controlled dimming when lighting is augmented by daylight to match the sunlight contribution and individual task-lighting control.
Another concept of dimming that is growing in significance as energy conservation becomes increasingly important involves automatic energy control for constant illumination. As pointed out previously with respect to conventional lighting design approaches, an illumination system designed with the capability of more footcandles than initially required in order to make up for lamp lumen depreciation, dirt effects and other light-loss factors will initially provide more light and use more power than actually required. With automatic energy control, illumination can be reduced initially to the specified minimum maintained level, then slowly adjusted up during the time of use as the various light-loss factors develop until 100% lamp output is reached at relamping. Automatic energy control, however, requires an effective power control circuit for lamp dimming and an accurate, reliable means for adjusting the power control circuit.
Only recently, however, have lighting system components for effectively dimming HID lamps been developed. A HID lamp dimmer is revealed in U.S. Pat. No. 3,894,265, filed Feb. 11, 1974 and entitled "High Intensity Lamp Dimming Circuit", assigned to the same assignee as the present application and hereby incorporated by reference. Although an effective dimmer circuit has been developed, the realization of automatic energy control has not come into being for lack of development of an accurate and reliable automatically variable d.c. voltage source for controlling such a HID lamp dimmer circuit.